The present invention relates principally to a method and apparatus for use in association with a printing press or other web processing equipment where upon a break in the web it is necessary to effect a rapid shut-down of the equipment.
Web processing equipment is commonly utilized in a variety of industries to, for example print or otherwise process paper, fabric, plastic film or foil and the like. In such applications the web is moved continuously through the equipment at high rates of speed from the input to the output of the equipment so that in the event of a break in the web it is necessary to stop the equipment, or at least the web feed to the equipment, at the earliest possible time. Rapid response to a web break is necessary to prevent excessive loss of web material since material in the equipment at the time of the break and material fed to the equipment immediately after the web break is unusable. Also continued operation of the equipment after a break can damage the equipment and cause excessive downtime to cleanup, repair equipment and remove the unusable web from the equipment.
The prior art teaches various types of web sensing equipment to detect web breakage and thus the absence of the web material.
Such prior art devices include mechanical arrangements which may include pivotal rollers with connectors to actuate alarm devices and/or a web cutting knife in the event of a break in the web.
Other prior art arrangements have included photoelectric devices where a beam of light, either visible or infrared, is directed transversely across the web path to a photosensitive cell so that when the web is in place the cell does not "see" the light beam and when the web breaks the cell is exposed to the light beam to initiate corrective action. The infrared device can also operate in such a way that the beam is reflected off of the web back to a receiver contained in the transmitter package. When the web is in place the receiver receives a reflected signal. When the web breaks the receiver, loses the signal, and corrective action is initiated.
It has been found that such infrared devices, however, in many such applications, and particularly in printing applications, the web processing generates a heavy dust load and in many situations ink mist is heavy in areas surrounding the equipment. Therefore the environment in which these prior art devices have operated have tended to diminish the effectiveness of the devices because the source and receiver have become coated with dust and/or ink so that the devices in many instances do not function properly.
Also these prior art devices can only detect the presence or absence of a web and therefore can not detect an excessive slack in the material being processed. In the operation of, for example, a printing press, this slack in the web material could cause a wrap-up and thus damage the equipment.
In operation many prior art devices have either stopped the operation of the web drive machine, sounded an alarm, or cut the web of the infeed inresponse to a detected break in the web.
One prior art arrangement utilizing web break detectors is shown in U.S. Pat. No. 3,928,844-Melhofer where a web break detector system unlike that in the present invention is illustrated wherein numerous such devices are utilized in co-operative relation to detect the disappearance of a web from a station.
Another arrangement is shown in U.S. Pat. No. 4,078,487-McComb.
No prior art device is known which utilizes an ultra-sonic sound generating source and receiving source to receive a reflected ultra-sonic signal in order to detect a web break and/or excessive slack in a web processing device.